The disclosure relates to blade rub coatings. More particularly, the disclosure relates to abradable coatings for turbomachines such as gas turbine engines.
Abradable coatings (rub coatings) protect moving parts from damage during rub interaction and wear to establish a mating surface to the moving parts with smallest possible clearance. The coatings are used in turbomachines to interface with the tips of a rotating blade stage, tips of cantilevered vanes and knife edge seals.
In an exemplary turbomachine such as a gas turbine engine, more particularly, a turbofan engine, coatings may be used to interface with the blade tips of fan blade stages, compressor blade stages, and turbine blade stages. Because temperature generally increases through the fan and compressor and is yet much higher in the turbine, different blade materials, surrounding case materials, and coating materials may be desired at different locations along the engine.
With relatively low temperatures in the fan and compressor sections, relatively low temperature materials may be used for their blades and the surrounding cases (at least through upstream (lower pressure) portions of the compressor). The exemplary blade materials in such lower temperature stages may be aluminum alloy, titanium alloy, carbon fiber or other composite, combinations thereof, and the like. Similarly, relatively lower temperature case materials may be provided. Particularly because the case material is not subject to the centrifugal loading that blades are, even lower temperature capability materials may be used (e.g., aramid or other fiber composites) in the case than in the blades.
It is known to use a coating along the inboard or inner diameter (ID) surface of the case component to interface with the blade tips. Such coatings serve to protect blade tips from damage during rub contact between the blades and case. When the blade tips are protected from damage during rub, clearance between the blades and case ID can be set closer and tighter operating clearance can be achieved.
To limit blade damage, the adjacent surfaces of the surrounding shroud may be formed by an abradable rub coating. Examples of abradable rub coatings are found in U.S. Pat. Nos. 3,575,427, 6,334,617, and 8,020,875. One exemplary baseline coating comprises a silicone matrix with glass micro-balloon filler. Without the glass filler, the elastic properties of the abradable coating result in vibrational resonances and non-uniform rub response. The glass increases the effective modulus of the coating so as to reduce deformation associated with aerodynamic forces and resonances.